This invention relates to light emitting diode (LED) lighting systems for producing white light, and in particular to hybrid LED lighting systems for producing white light comprised of LEDs and phosphor-LEDs. The hybrid lighting system exhibits improved performance over conventional LED lighting systems that use LEDs or phosphor-LEDs to produce white light.
Conventional LED lighting systems for producing white light typically comprise either LEDs or phosphor-LEDs. Lighting systems which use LEDs produce white light by combining various combinations of red, green, and blue LEDs. Phosphor-LED based lighting systems produce white light by using one or more various luminescent phosphor materials on top of a blue light LED to convert a portion of the emitted blue light into light of a longer wavelength.
Lighting systems which use LEDs to produce white light are more efficient at the package level than lighting systems which use phosphor-LEDs. However, high quality white light is more difficult to achieve in LED based lighting systems. This is because LEDs manufactured to optimize total lighting system performance and production typically must be combined in an undesirably large integral number of LED chips to provide the requisite quantities of red, green and blue light when operated at full rated power. Moreover, the LED chips must be fabricated in different sizes to achieve the proper balance thus, increasing the production costs of the system. Since the green and blue LED chips are manufactured in the same AlInGaN technology, there are fabrication and cost advantages to making these chips the same size, and reasonably large.
There are other limitations associated with LED based lighting systems. Existing green LEDs operating at the very desirable light spectral wavelength of about 550 nm are very inefficient. The highest luminous-efficacy green LED operates at a less desirable light spectral wavelength of about 530 nm. Further, currently available efficient LEDs make good color rendering difficult to achieve. Good color rendering is possible, but places constraints on specific choices of LEDs.
Additionally, mixing LEDs to produce white light entails material and particularly efficiency costs. More specifically, many highly collimated mixing schemes are binary in that they mix two LEDs at a time. LED based lighting systems typically use three and four LEDs and thus, require two stages of mixing. Unfortunately, each stage of mixing has an efficiency cost which significantly lowers the performance of the system.
As alluded to earlier, it is easier to produce white light with phosphor-LED based lighting systems as compared with LED based lighting systems because phosphor-LEDs do not require mixing and have lower material costs (they are inherently mixed). However, they are less efficient by a factor of about two at the package level than LED based lighting systems because of quantum deficits and re-emission efficiencies.
Accordingly, there is a need for a lighting system which combines certain aspects of LED and phosphor-LED based lighting systems to achieve benefits beyond either system.
Accordingly, we disclose herein a lighting system for producing white light that includes at least one LED and a phosphor-LED disposed adjacent to the at least one LED.